Wires or cables used as superconductors typically are composed of a composite of multiple superconducting filaments embedded in a comparatively large matrix of material that is a normal conductor. Copper and aluminum are examples of common matrix materials. The matrix acts as a protective shunt for preventing the composite conductor from being damaged on accidentally passing from the superconductive state into the normal (ohmic or resistive) state, as superconductive materials are ordinarily poor conductors under non-superconducting conditions. If a superconducting system lacks a matrix and suddenly becomes non-superconductive, the heat dissipated by the now-resistive "superconductors" burns out the filaments. When the composite superconductor is operating under conditions where it is in the normal state, the matrix carries the electrical current temporarily. Under superconducting conditions, the superconductive filaments conduct the electrical current.
Where it is necessary to splice or connect the ends of two or more of these multifilamentary composite superconductors together, the resultant joints must be both shunted and superconductive. If the joint is not shunted, it has the same potential to burn up as the individual filaments do without a matrix. The joint must be superconductive to maintain the persistent mode along the length of the superconductor since otherwise the joint acts as a resistor in the circuit. Since the current carried by the superconducting filaments is ordinarily quite large, significant energy would be dissipated in a resistive joint, even if the joint were not destroyed.
A superconducting joint must generally withstand exceptionally large mechanical and electrical stresses, particularly where such multifilamentary superconductors are used as coil circuits for the purpose of creating high strength, uniform magnetic fields. To achieve such magnetic fields, the circuits must carry current at a high level of current density, typically 10,000 amperes/cm.sup.2 or more. In such high field magnets, the mechanical stress in the wire due to the field increases as the square of the field.
A superconductive joint should thus preferably be mechanically strong and durable, maintain a persistent mode, and have good shunting capacity. The prior art has sought to accomplish these ends by forming joints using a process of either welding or crimping, or a combination of both. In one method, the superconducting filaments are welded to an intermediate foil after stripping the ends of the surrounding matrix. This method, however, is impractical when hundreds or thousands of filaments are to be joined. A second method is to crimp the ends together in a sleeve made of copper. This method of joint formation can be difficult to repeat with high reliability.
In a method disclosed in U.S. Pat. No. 4,631,808 to Jones, one or more superconductive conductors embedded in a resistive matrix are joined by removing the resistive matrix from a length of superconductive composite conductors to form exposed ends of superconductive filaments, and then juxtaposing the exposed filaments in a sleeve of superconducting material and then crimping the sleeve about the exposed filaments. Thermal fusion of the exposed ends of the conductors may then be performed, if necessary. A final step is then taken in which the crimped joint is encapsulated in a conductive material such as indium that has a low melting point but is solid at the cryogenic temperatures employed. The encapsulation is meant to act as a shunt current path and to provide physical strength and rigidity to the structure. The crimped sleeve, which forms the outer surface of the crimped joint, is difficult to solder because it is made from superconducting materials which do not readily bond to either superconducting or normal conducting materials. The result is that bonding may be poor between the sleeve and the encapsulating material, and between the sleeve and the exposed ends of the superconductive filaments, which can result in loss of effectiveness of the connector over prolonged use. The joint may also be susceptible to damage if the system accidentally shifted to normal conduction. The joint is shunted by directing current in the resistive mode from the encapsulating material to the matrix of the two superconductors that are being connected. If the superconductive sleeve is not properly encapsulated by the conductive material, or if the superconductive sleeve extends too far over the matrix, then the joint will be unable to properly shunt the current.